Ink jet printers employ pens having print heads that reciprocate over a media sheet and expel droplets onto the sheet to generate a printed image or pattern. A typical print head includes a silicon chip substrate having a central ink hole that communicates with an ink filled chamber of the pen when the rear of the substrate is mounted against the pen. An array of firing resistors is positioned on the front of the substrate, within a chamber enclosed peripherally by a barrier layer surrounding the resistors and the ink aperture. An orifice plate connected to the barrier just above the front surface of the substrate encloses the chamber, and defines a firing orifice just above each resistor. Additional description of basic printhead structure may be found in "The Second-Generation thermal Inkjet Structure" by Ronald Askeland et al. in the Hewlett-Packard Journal, August 1988, pages 28-31; "Development of a High-Resolution Thermal Inkjet Printhead" by William A. Buskirk et al. in the Hewlett-Packard Journal, October 1988, pages 55-61; and "The Third-Generation HP Thermal Inkjet Printhead" by J. Stephen Aden et al. in the Hewlett-Packard Journal, February 1994, pages 41-45.
For a single color pen, the resistors are arranged in two parallel elongated arrays that each extend nearly the length of the substrate to provide a maximum array length for a given substrate chip size. The resistor arrays flank opposite sides of the ink aperture, which is typically, an elongated slot or elongated array of holes. To ensure structural integrity of the substrate, the ink aperture does not extend too close to the substrate edges, or as close to the edges as the endmost several firing resistors. Therefore, several resistors at each end of each array extend beyond the end of the ink supply aperture or slot.
While a reasonably effective configuration, it has been found that the end firing elements, that is, those that include the end resistors, are more susceptible to failure than are the multitude of firing elements that adjoin the length of the ink supply slot. It is believed that small air bubbles come primarily from two sources: those that arise from outgassing of ink components during normal operation, and those left behind after completion of pen assembly. These bubbles tend to aggregate and coalesce into larger bubbles in ends of the ink chamber. This occurs in the portions beyond the ends of the ink supply slots, and in the vicinity of the end resistors. Small bubbles present are normally tolerated because they can usually be "ejected," with only a single ink droplet being omitted from printed output; the firing element then continues properly following the momentary tolerable failure. However, it is believed that when the small tolerable bubbles are permitted to coalesce, they become large enough to permanently block one or more firing elements, preventing ink from reaching a firing, resistor.